Fluorinated ion exchange polymers having carboxylic acid and/or sulfonic acid functional groups or salts thereof are known in the art. One principal use of such polymers is as a component of a membrane used to separate the anode and cathode compartments of a chloralkali electrolysis cell. Such membrane can be in the form of a reinforced or unreinforced film or laminar structure.
Experience over recent years has shown that electrochemical cells, especially chloralkali cells, exhibit best performance when the membrane employed in such cells is one having a multilayered structure wherein the layers are of diverse composition and serve different functions. For example, for a chloralkali cell, a membrane having a layer of a sulfonate polymer, which has a low electrical resistance but poorly rejects passage of hydroxyl ions, and a layer of a carboxylate polymer, which has a higher electrical resistance but rejects hydroxyl ions more efficiently, provides better cell performance than a membrane of sulfonate polymer only or carboxylate polymer only. Such structures may have two, three, or even more layers.
However, preparation of such multilayered membranes is a complex art, in which such structures are built up by thermal lamination of component films, coextrusion of different polymers, or chemical modification of one or more layers of a preformed membrane, or a combination of these procedures. Such procedures are ordinarily performed on films having functional groups which are not in ion-exchange form, and so must be subsequently converted to ion-exchange form, with attendant problems such as inadequate adhesion of the layers after performing steps such as hydrolysis. Avoidance of such complex preparation is highly desirable. Furthermore, scrap produced in such complex preparative methods is often a total loss, inasmuch as the components thereof can be recovered separately only with great difficulty or not at all; scrap which cannot be recycled or recovered is a highly significant loss in this field in view of the high cost of the fluorinated components.
In East German Patent Specification No. 76,478 and British Patent Specification No. 1,273,164, which are in the names of the same inventors, there is disclosed a combination of two cation exchanger films arranged next to one another in a chloralkali cell. As pointed out in East German Patent Specification No. 93,990, a disadvantage to such arrangement is that the two films employed have very different swelling properties, as a result of which distortions arise across the entire surface thereof, spaces filled with electrolyte form between the two films, and undesired polarisation phenomena occur which adversely affect the economics of the electrolysis. Further, it would be desirable to attain even greater current efficiency than the 85% disclosed and attained in East German No. 76,478.
It is therefore a principal object of this invention to provide a multilayered membrane which is more simply made than by heretofore known methods, and by a method which does not require high investment in machinery.
It is a further object to provide multilayered membrane made such that production of scrap from which recovery of the component materials is difficult is avoided.
It is a still further object to provide a multilayered membrane wherein the non-adhered layers thereof do not tend to separate from one another during use in electrolysis of brine in a chloralkali cell, and which provides high current efficiency.